In a microscope, the objective (sometimes referred to in the art as an objective lens) is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. For example, the objective lens of a microscope is the one at the bottom near the sample. At its simplest, it is a very high-powered magnifying glass, with very short focal length. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube. The microscope objective itself is typically substantially cylindrical or tubular and contains one or more lenses, typically made of glass, confined within a protective barrel. Microscope objectives generally have an optical inlet and optical exit, typically centered along opposite ends of its longitudinal axis. The optical inlet and optical exit are connected by an optical path extending between them through the microscope objective.
Microscope objectives are typically characterized by two parameters: magnification and numerical aperture. The former typically ranges from 4×-100×, while the latter ranges from about 0.1 to 1.4, and focal lengths of about 30 millimeters to about 200 microns, respectively. Similarly, microscope objectives with numerical apertures in the range of about 0.1 to 1.4 typically have respective working distances (i.e., the distance between the microscope objective and the focal point where imaging occurs) of from several millimeters to about 210 microns. Similarly, for high magnification applications, an oil-immersion objective or water-immersion objective generally has to be used. The objective is specially designed to use refractive index matching oil or water to fill the air gap between the front element and the object to allow the numerical aperture to exceed 1, and hence give greater resolution at high magnification. Numerical apertures as high as 1.5 or even higher can be achieved with oil immersion. Microscope objectives with high numerical aperture (NA) and of high quality are typically quite expensive.
Microscope objectives are also used to focus laser light in a process known as multiphoton stereolithography. In that process, laser light (typically in the infrared) is focused in a polymerizable composition commonly termed a “photoresist”, typically supported on a substrate. The photoresist contains a multiphoton absorbing compound, and the laser has sufficiently high power that two (or, less typically, more than two) photons are absorbed essentially simultaneously by the multiphoton absorbing compound resulting in subsequent polymerization of the photoresist.
In order to improve resolution, one conventional approach has been to partially submerge the objective lens assembly into a liquid photoresist to eliminate the air interface/objective lens interface. However, the localized laser power in multiphoton stereolithography can be considerable, and the potential exists for a buildup of polymerized photoresist material over time on the microscope objective lens that could be difficult to remove from the optical surfaces. Were this to happen, there is a potential that an expensive microscope objective could be rendered unusable.